Experimental ECC threshold decryption to replace PBC threshold decryption

[Vagabond]

This is currently experimental. It replaces the threshold decryption with a combination of Shamir's secret sharing of an AES key and ECC signatures/encryption over the key shares.

There's a lot to do before this can merge:

• Make all the tests pass
• Make this scheme optional and restore the old threshold decryption
• Use a Key Derivation Function like HKDF to strengthen the ECDH keys
• Consider adding an extra 'key reveal' message if a node has enough correct shares to decrypt (to avoid a poison share preventing some nodes from decrypting)
• Profile it to look for further optimizations